Synthesis of tetramethylammonium hydroxide

ABSTRACT

In order to prepare tetramethylammonium hydroxide by electrolysis of a tetramethylammonium salt in a cell with a cation-exchange membrane, the operation is carried out continuously under stationary conditions obtained, on the one hand, by the introduction, into the anode electrolysis loop, of a tetramethylammonium salt solution which is more concentrated than that present in the cell and by an input of water into the cathode loop and, on the other hand, by the withdrawal of a portion of each of the solutions circulating in the anode and cathode loops.

[0001] The present invention relates to tetramethylammonium hydroxide and has more particularly as subject-matter the synthesis of this compound by a continuous electrolysis process under stationary conditions.

[0002] Tetramethylammonium hydroxide (TMAH), which is one of the most widely used products in the electronics industry (developing, etching, planarizing and photoresist stripping), is obtained in practice in two stages, namely first the synthesis of a tetramethylammonium salt and subsequently the conversion of this salt to the hydroxide by electrolysis.

[0003] According to Patents JP 57-155390, U.S. Pat. No. 4,634,509 and U.S. Pat. No. 4,776,929, this electrolysis stage is carried out in an electrochemical cell comprising two compartments separated by a cation-exchange membrane and two electrodes, with an oxidation reaction on the anion of the tetramethylammonium salt at the anode, reduction of water at the cathode and transfer of the tetramethylammonium cation (TMA⁺) through the membrane. The processes disclosed in the abovementioned patents all proceed in the same way: filling the anode circuit with a concentrated solution of TMA⁺ salt, filling the cathode circuit with deionized or demineralized water comprising from 0.1 to 1% of TMAH, in order to ensure minimum conductivity, and then beginning the electrolysis. This way of operating is inevitably discontinuous because the TMAH crystallizes at 50 weight % in the form of a pentahydrate and because, after a certain time, which is dependent on the volume of the loop and on the current, there are no longer TMA⁺ ions in the anode circuit.

[0004] This way of operating exhibits several disadvantages:

[0005] 1) The catholyte is initially not very conducting, resulting in a high ohmic drop. During electrolysis, its conductivity will increase but that of the anode compartment will decrease in parallel. Overall, the ohmic drop of the system is therefore always very high, which is highly disadvantageous to the cell voltage, which fluctuates between 7 and 11 V for a current density of 1 kA/m² and between 15 and 23 V for a current density of 2 kA/m². This high ohmic drop can result in a significant increase in the temperature via the Joule effect.

[0006] 2) The concentrations of TMAH and of TMA⁺ salt (for example the chloride) are continually changing. The membrane therefore never operates under stationary conditions, which is harmful to its lifetime and finally leads to a fall in the yield and in the quality of the TMAH synthesized.

[0007] In order to overcome these disadvantages, the present invention now provides a process for the synthesis of TMAH by continuous electrolysis of a TMA⁺ salt in a cell with a cation-exchange membrane under stationary conditions, that is to say the parameters of the electrolysis cell, in particular the concentrations of the various solutions, remain stable over time for a fixed current density.

[0008] The process according to the invention for the preparation of TMAH by electrolysis of a tetramethylammonium salt in a cell with a cation-exchange membrane is characterized in that the operation is carried out continuously under stationary conditions obtained, on the one hand, by the introduction, into the anode electrolysis loop, of a tetramethylammonium salt solution which is more concentrated than that present in the cell and by an input of water into the cathode loop and, on the other hand, by the withdrawal of a portion of each of the solutions circulating in the anode and cathode loops.

[0009] These two means (introductions and withdrawals) make it possible to guarantee optimum operation of the membrane of the electrolysis cell and thus to obtain better performances and to retain them over time. It also results from this that the cell voltage is constant over time (for example, 7-11 V for a current density of 3 kA/m²), at a value far below those indicated in the abovementioned patents. The membrane then operates continuously under optimum conditions, which results in high and stable values of the current efficiencies, stability of the pH in the anode compartment and limited side reactions. All this makes it possible to optimize the lifetime of the membranes, to obtain a constant quality for the TMAH synthesized and to minimize the energy consumption, for example approximately 3000 kWh/t (TMAH) continuously at 3 kA/m², against at best 4700 kWh/t (TMAH) at 2 kA/m² in the prior art.

[0010] The process according to the invention applies both to the synthesis of technical TMAH and to the synthesis of electronic-grade TMAH. Use is preferably made, as starting TMA⁺ salt, of tetramethylammonium chloride (TMA—Cl), tetramethylammonium hydrogencarbonate (TMA—HCO₃) or tetramethylammonium hydrogensulphate (TMA—HSO₄). The anode reaction corresponding to each of these salts is given in the following table: Nature of the salt Corresponding anode reaction TMA-Cl Cl⁻ → ½Cl₂ + e⁻ TMA-HCO₃ HCO₃ ⁻ → CO₂ + ¼O₂ + ½H₂O + e⁻ TMA-HSO₄ H₂O → ½O₂ + 2H⁺ + 2e⁻

[0011] The preferred anodes are based on platinum or on a ruthenium, iridium or platinum oxide.

[0012] The production of hydroxide ions at the cathode can take place either by reduction of water to OH⁻ ions and to hydrogen or by reduction of oxygen with water to OH^(—) ions.

[0013] The appended FIG. 1 represents the block diagram for the electrolysis in the case of operating with a cathode for reduction of water and release of hydrogen, this cathode preferably being made of stainless steel or of nickel. The device according to FIG. 1 comprises:

[0014] an electrolysis cell composed of an anode compartment (1) and of a cathode compartment (2) separated by a cation-exchange membrane (m)

[0015] an anode degassing tank (3) for removing, via the pipe (9), the gas generated by the anode reaction,

[0016] a cathode degassing tank (4) for removing, via the pipe (10), the hydrogen generated by the cathode reaction,

[0017] a tank (5) for storing the more concentrated solution of tetramethylammonium salt to be introduced into the anode loop via the pipe (5′),

[0018] a tank (6) for storing the demineralized or deionized water to be introduced into the cathode loop via the pipe (6′),

[0019] a tank (7) for emptying, via the pipe (7′), a portion of the tetramethylammonium salt solution exiting from the anode degassing tank (3),

[0020] a tank (8) for storing the solution of synthesized TMAH, which solution is drawn off from the cathode loop at the outlet of the cathode degassing tank (4) via the pipe (8′).

[0021] As illustrated in FIG. 2 in the case of the electrolysis of TMA—Cl to TMAH, the electrolysis cell operates according to the following principle:

[0022] oxidation of the chloride ion to chlorine at the anode according to the reaction:

2Cl⁻→Cl₂+2e⁻

[0023] reduction of water to hydrogen and hydroxide ions at the cathode according to the reaction:

2H₂O+2e⁻→H₂+2OH⁻

[0024] transfer of the TMA⁺ ion through the cation-exchange membrane, accompanied by a number of water molecules (number variable according to the nature of the membrane and the current density),

[0025] separation by the cation-exchange membrane of the anolyte, of the catholyte and of the gases produced.

[0026] The cathode loop and the storage of the TMAH produced can be rendered inert with hydrogen, nitrogen, argon or a mixture of these gases. In this case, the device used in the case of operating with a cathode for reduction of water and release of hydrogen is modified as shown in FIG. 3, where the region which has been rendered inert is represented in dotted lines, the inerting gas being introduced via the pipe (10) and the hydrogen generated by the cathode reaction being discharged via the pipe (11) exiting from the tank (8) for storage of the solution of synthesized TMAH.

[0027] The block diagram for the electrolysis in the case of operating with a cathode for the reduction of oxygen (preferably a cathode based on platinized or silvered carbon) is represented in the appended FIG. 4, where the cathode compartment (2) is fed with oxygen via the pipe (12) and where the pipe (10) acts as bleed for the oxygen. As illustrated in FIG. 5 in the case of the electrolysis of TMA—Cl to TMAH, the electolysis cell then operates according to the same principle as above, except that the reduction of water to hydrogen and hydroxide ions at the cathode is replaced by a reduction of oxygen with water to hydroxide ions according to the reaction:

O₂+2H₂O+4e⁻→4OH⁻

[0028] The optional inerting can in this case be carried out with oxygen, nitrogen, argon or a mixture of these gases.

[0029] The two electrodes can be pressed against the membrane (so-called “zero gap” arrangement) or the cathode can be placed a few millimeters from the membrane (so-called “finite gap” arrangement).

[0030] In the electrolysis cell, the important component is the membrane, since it is this which will ensure good separation of the two solutions. In order to guarantee good current efficiencies and a high purity of the synthesized TMAH, it must be permeable to the TMA⁺ ions but impermeable to the anions of the starting salt (Cl⁻, HCO₃ ⁻, and the like) and to OH⁻. Furthermore, in order to separate an acidic medium, indeed even a weakly basic medium, from a very basic medium (TMAH), it must be chemically stable in both media. Moreover, it must be as conductive as possible in order to minimize the ohmic drop. To meet all these criteria, ion-exchange membranes are generally composed of at least two layers of polymers, these layers generally being colaminated together. These polymers can be composed of perfluorosulphonated and/or perfluorocarboxylated chains. Such membranes are disclosed, for example, in Patents U.S. Pat. No. 4,401,711, EP 165,466, U.S. Pat. No. 4,604,323, EP 253,119 and EP 753,534. They are found commercially in particular under the names Nafion® N324, N902 and N966 from DuPont de Nemours, Flemion® 892 and 893 from Asahi Glass or Aciplex® 4203 from Asahi Chemicals.

[0031] In order to obtain an identical degree of swelling of the various polymers and thus to avoid a deterioration in the membrane (for example, by delamination of the layers), it is necessary to adjust the concentrations of the anolyte and of the catholyte. This is because this deterioration in the membrane would result, on the one hand, in a loss in performance of the cell, since the OH⁻ ions can be oxidized at the anode to form oxygen, and, on the other hand, in a decrease in the purity of the synthesized TMAH, since the delamination of the membrane makes possible a flow of the TMA⁺ salt solution into the TMAH.

[0032] The process according to the invention is advantageously carried out under the following conditions:

[0033] current density of between 1 and 5 kA/m², preferably between 3 and 4 kA/m²

[0034] temperature of between room temperature and 80° C., preferably between 40 and 60° C.

[0035] concentration of TMAH in the cathode loop of between 5 and 40% by weight, preferably between 10 and 25%

[0036] concentration of tetramethylammonium salt in the anode loop of between 15 and 40% by weight, preferably between 20 and 35%.

[0037] The input of TMA⁺ salt and of water via the concentrated solution of TMA⁺ salt introduced into the anode loop is determined so as to compensate for the consumptions of TMA⁺ salt and of water related to the electrochemical reaction at the anode and to the transfers through the membrane. Likewise, the input of water into the cathode loop must contribute, with the water transferred from the anode compartment to the cathode compartment through the membrane, to compensating for the water consumed by the electrochemical reaction at the cathode and to providing the water required in order to obtain the desired final concentration of TMAH. These inputs and concentration of TMA⁺ salt depend in particular on the nature of the membrane used, on the current density chosen, on the area of the electrodes and on the desired concentration of TMAH.

EXAMPLES

[0038] The following examples, which illustrate the invention without limiting it, were produced by means of the experimental device represented in FIG. 6.

[0039] The cell is composed of two independent circuits, one an anode circuit and the other a cathode circuit:

[0040] The anode circuit is composed of a PTFE-base compartment which allows the anolyte to circulate in the electrochemical cell and which comprises the anode (made of titanium coated with RuO₂—TiO₂). This compartment is connected to an anolyte/gas degassing column in which the addition of concentrated TMA⁺ salt solution and the discharge of the depleted anolyte are also carried out. During electrolysis, the gas generated at the anode is discharged via the back of the electrode and the circulation of the anolyte is carried out by “gas lift” (difference in relative density between the two-phase mixture and the solution). The temperature is adjusted using a heating tape surrounding the degassing column.

[0041] The cathode circuit is symmetrical with the anode compartment and is based on the same operating principle. The cation-exchange membrane is placed between the anode and the cathode. The anode is pressed against the membrane and the cathode, composed either of a nickel grid or of a stainless steel plate pierced with small holes for the discharge of the gases, is placed 4 millimeters from the membrane (“finite gap” arrangement) . The input of demineralized water and the discharge of the synthesized TMAH take place in the degassing column.

[0042] The working area of the cell is 50 cm². The material used for the electrochemical cell and the various pipes is of PTFE and, for the fittings (columns and tanks), of polypropylene.

[0043] The entire cathode loop is rendered inert with a mixture of hydrogen (generated at the cathode) and of argon (injected), in order to limit the dissolution of atmospheric CO₂ in the TMAH.

[0044] The circulation of the electrolytes takes place by a difference in relative density due to the gas releases (chlorine or CO₂ at the anode, depending on the starting salt, and hydrogen at the cathode).

[0045] The water injected into the cathode circuit in order to maintain the concentration of TMAH is distilled water.

[0046] The chlorine produced during the tests from TMA—Cl is destroyed in a scrubbing column (not represented) using sodium hydroxide and sodium sulphite.

[0047] The TMAH storage bottle is sealed and equipped with a drawing-off/emptying valve in the bottom part. Two non-return bottles, one comprising water and the other empty, make it possible to prevent contamination by the outside atmosphere. Samples are taken under a controlled atmosphere (argon or nitrogen) in a glove box.

[0048] The cell is started up according to the following protocol:

[0049] filling the anode compartment with an aqueous solution of the TMA⁺ salt at the operating concentration of the electrolysis,

[0050] filling the cathode compartment with an aqueous TMAH solution at the operating concentration of the electrolysis,

[0051] inerting the cathode circuit by purging with argon (if it is desired to limit the dissolution of atmospheric CO₂ in the TMAH, in particular for a product for the electronics industry),

[0052] switching on the heating tapes, in order to bring the device to the desired temperature, and gradual raising of the current density.

Example 1

[0053] The electrochemical cell is equipped with an anode formed of RuO₂—TiO₂ deposited on expanded titanium, a cathode made of stainless steel (perforated plate) and a Nafion® N324 membrane preconditioned by immersion in a 10% TMAH solution for 24 hours. This cation-exchange membrane is a membrane with perfluorosulphonated chains sold by DuPont de Nemours.

[0054] The anode circuit is filled with 735 g of a 243 g/l aqueous TMA—HCO₃ solution. The cathode circuit is filled with 780 g of a 237 g/l aqueous TMAH solution. The entire assembly is heated with heating tapes and the cathode circuit is rendered inert by injection of argon. When the temperature of the fluids reaches 50° C., the electrical supply is switched on and the current is increased by 1 A every 3 minutes until 15 A is reached, i.e. 3 kA/m².

[0055] The input of water is 100 g/h and the input of TMA—HCO₃ is 125 g/h (588 g/l solution).

[0056] After operating for 16 hours under these conditions, the following results were obtained:

[0057] a) The concentration of TMAH is 244 g/l in the storage tank and 235 g/l in the degassing column of the cathode circuit. The current efficiency for the cathode reaction is 94%.

[0058] b) The concentration of TMA—HCO₃ is 247 g/l in the emptying tank and 260 g/l in the anode degassing column. The current efficiency for the anode reaction is 97%.

[0059] c) The voltage in the electrolysis cell remains stable at 3 kA/m at a value of 10 V, i.e. an energy consumption of 3138 kWh/t (TMAH).

Examples 2 to 6

[0060] Other examples according to the invention were produced by proceeding as in Example 1 but by using other materials (membrane, cathode) and/or by starting from another tetramethylammonium salt (TMA—X, X denoting the anion) and/or by varying the concentrations of TMA—X and of TMAH.

[0061] Nafion® N902 and N966 membranes are cation-exchange membranes sold by DuPont de Nemours.

[0062] The operating conditions and the results obtained after operating for 16 hours are summarized in the following table, in which:

[0063] E_(cell) denotes the cell voltage (V) and

[0064] η denotes the current efficiency, that is to say the ratio of the current portion which has actually been used to carry out the desired reaction to the total current used, η_(a) representing the current efficiency for the anode reaction (oxidation of chloride ions to Cl₂ or of hydrogencarbonate ions to CO₂) and η_(c) representing the current efficiency for the cathode reaction (synthesis of hydroxide ions).

[0065] The energy consumption (W) of the electrolysis cell, expressed in kWh/tonne of TMAH, can be calculated from the data in the preceding table by the formula W=295×E_(cell)/η_(c). Example 1 2 3 4 5 6 Membrane N324 N966 N966 N324 N902 N324 Cathode Stain- Nickel Stain- Stain- Nickel Nickel less less less steel steel steel X⁻ anion HCO₃ ⁻ Cl⁻ Cl⁻ Cl⁻ Cl⁻ HCO₃ ⁻ [TMA-X], 243 g/l 254 g/l 203 g/l 249 g/l 200 g/l 320 g/l starting [TMA-X], 588 g/l 505 g/l 580 g/l 533 g/l 250 g/l 602 g/l input [TMA-X], 247 g/l 243 g/l 191 g/l 235 g/l 176 g/l 332 g/l emptying [TMAH], 237 g/l 106 g/l 107 g/l 247 g/l 100 g/l 244 g/l starting [TMAH], 244 g/l 105 g/l 105 g/l 235 g/l 105 g/l 253 g/l storage η_(a) 97% 97.5% 98% 94% 95.4% nd^((a)) η_(c) 94% 93% 92.5% 95% 91.8% 92% E_(cell) 10 V 9 V 8 V 11 V 7 V 10 V

Comparative Examples 7 to 10

[0066] Four examples, produced by operating with the same device but under nonstationary conditions with regard to concentration at TMA—X (Example 10) or with regard to concentrations of TMA—X and TMAH (Examples 7 to 9), are summarized in the following table. Examination of the results obtained shows that the efficiencies are much lower than in Examples 1 to 6 in accordance with the invention. Example 7 8 9 10 Membrane N902 N902 N902 N902 Nickel Stainless Stainless Nickel Cathode steel steel X⁻ anion Cl⁻ Cl⁻ Cl⁻ HCO₃ ⁻ [TMA-X], 247 g/l 328 g/l 248 g/l 335 g/l starting [TMA-X], 247 g/l 328 g/l 248 g/l 335 g/l input [TMA-X], 144 g/l 270 g/l 160 g/l 222 g/l final^((b)) [TMAH],  50 g/l 250 g/l 248 g/l 107 g/l starting [TMAH], 203 g/l 320 g/l 159 g/l 106 g/l final^((b)) Duration of 8 h 8 h 8 h 4 h the test η_(a) 84% 59% 82% 86% η_(c) 80% 59% 78% 89% E_(cell) 7.5 V 7 V 7 V 10 V

[0067] (b) As the operating conditions in these comparative examples are not stationary, the terms “[TMA—X], final” and “[TMAH], final” are understood here to mean the concentrations of the solutions present in the degassing columns at the end of the test.

[0068] It is to be understood that, although specific examples have been given, the present invention is not restricted in view thereof, but can be practiced with many modifications and variations by those skilled in the art without departing from the spirit and scope of the invention.

[0069] The articles listed herein forms a part of this application and is incorporated herein by reference. 

1. Process for the preparation of tetramethylammonium hydroxide by electrolysis of a tetramethylammonium salt in a cell with a cation-exchange membrane, characterized in that the operation is carried out continuously under stationary conditions (concentrations of the solutions in the cell for a fixed current density) obtained, on the one hand, by the introduction, into the anode electrolysis loop, of a tetramethylammonium salt solution which is more concentrated than that present in the cell and by an input of water into the cathode loop and, on the other hand, by the withdrawal of a portion of each of the solutions circulating in the anode and cathode loops.
 2. Process according to claim 1 , in which the tetramethylammonium salt is the chloride, the hydrogencarbonate or the hydrogensulphate.
 3. Process according to claim 1 or 2 , in which the operation is carried out with a cathode for the evolution of hydrogen.
 4. Process according to claim 3 , in which the cathode is made of stainless steel or of nickel.
 5. Process according to claim 1 or 2 , in which the operation is carried out with a cathode for the reduction of oxygen.
 6. Process according to claim 5 , in which the cathode is based on platinized or silvered carbon.
 7. Process according to one of claims 1 to 6 , in which the anode is based on platinum or on a ruthenium, iridium or platinum oxide.
 8. Process according to one of claims 1 to 7 , in which the current density is between 1 and 5 kA/m², preferably between 3 and 4 kA/m².
 9. Process according to one of claims 1 to 8 , in which the operation is carried out at a temperature of between room temperature and 80° C., preferably between 40 and 60° C.
 10. Process according to one of claims 1 to 9 , in which the concentration of TMAH in the cathode loop is between 5 and 40% by weight, preferably between 10 and 25%.
 11. Process according to one of claims 1 to 10 , in which the concentration of tetramethylammonium salt in the anode loop is between 15 and 40% by weight, preferably between 20 and 35%.
 12. Process according to one of claims 1 to 11 , in which the cation-exchange membrane is composed of at least two layers of polymers with perfluorosulphonated and/or perfluorocarboxylated chains.
 13. Process according to one of claims 1 to 12 , in which the cathode loop and the storage of the tetramethylammonium hydroxide produced are rendered inert with hydrogen, nitrogen, argon or a mixture of these gases. 